1. Field of the Invention
The present invention relates to differential pressure flow instruments for determining the rate of fluid flow through a pipe and more specifically to orifice fittings.
2. Description of the Prior Art
The predominant measurement in the process industry is flow rate. The need for accuracy in the measurement of fluid flow in the process industry is extremely important. A small error in the flow rate measurement of large quantities of fluid through a pipe translates into a potentially huge loss of revenue.
Normally, whenever possible, flow rates are measured with a flat orifice plate which is installed in the pipe carrying the fluid. Typically, the flat orifice plate is housed in an orifice flow meter called an orifice fitting. The orifice plate has a circular bore whose diameter is less than the diameter of the pipe in which it is installed. As Venturi discovered in the late 1700s, fluids gain speed and lose pressure when flowing through a converging section of pipe. Thus, as the fluid converges and accelerates to flow through the smaller bore, it loses pressure. The loss of pressure becomes the differential pressure used to measure the rate of flow.
Many factors affect the accuracy of the readings and determination of the rate of flow using orifice fittings. Standards have been established and adopted by various professional associations. The most popular basis for orifice flow meter design and installation and the accepted standard of orifice flow meter design and installation for the natural gas industry and process industries in the United States and Canada is the American Gas Association (AGA) publication, "Orifice Measurement of Natural Gas, Gas Measurement Committee Report No. 3." The American Petroleum Institute (API) incorporated the AGA publication in its Manual of Petroleum Measurement Standards, Chapter 14--"Natural Gas Fluids Measurement" (hereinafter referred to as "API/AGA 14.3"). Among the many standards established in these publications is the maximum allowable orifice plate bore eccentricity .SIGMA.. The orifice plate bore must be concentric with both the upstream and downstream bore of the orifice fitting, although the eccentricity with respect to the upstream side is the most critical.
FIGS. 1 and 2 show one type of orifice fitting known as a "simplex" orifice fitting, generally designated as F. The simplex orifice fitting F shown in FIGS. 1 and 2 is representative of a typical simplex orifice fitting that is widely used to measure flow rates of fluids in pipelines, particularly in the petrochemical industry. Referring to FIGS. 1 and 2, the simplex orifice fitting F includes a circular, flat orifice plate 10 having a concentrically located bore 12 therethrough. The orifice plate 10 is held in place in a plate carrier 14 by a seal ring 16. The plate carrier 14 is securely mounted to a sealing bar 18 with a pair of bolts 20. A sealing gasket 22 is positioned adjacent to and between the sealing bar 18 and the plate carrier 14. As shown in FIG. 2, it is known in the prior art to include an alignment pin 30 received in opposing blind bores 14a and 18a of the plate carrier 14 and sealing bar 18, respectively. A line-up pin 26 extends upwardly from a sealing face 28 of the orifice fitting F. A line-up hole 18b in the sealing bar 18 is adapted to receive the line-up pin 26 during the installation of the plate carrier 14 into its operative position in the orifice fitting F as shown in FIG. 2.
Referring to FIGS. 1 and 2, the sealing bar 18 is further secured by a clamping bar 34 having a plurality of clamping bolts 36 which forcibly engage the sealing bar 18 into sealing contact with the sealing gasket 22 and the sealing face 28 of the simplex orifice fitting F.
Referring to FIG. 2, the bore 12 of the orifice plate 10 must be concentric or within an allowable eccentricity .SIGMA. with the longitudinal centerline 32a of the upstream and downstream longitudinal bore 32 (FIG. 1) of the simplex orifice fitting F.
As shown in FIG. 2, the operative position of the orifice plate bore 12 of the simplex orifice fitting F of FIGS. 1 and 2 is determined along the X-axis by the placement of the sealing bar 18 onto the line-up pin 26. The plate carrier 14 is rigidly secured to the sealing bar 18 via the bolts 20 and the alignment pin 30 as shown in FIG. 2. The operative position of the orifice plate bore 12 is determined along the Y-axis by the thickness of the sealing gasket 22 and the amount of compression experienced by the sealing gasket 22 resulting from the force applied by the plurality of clamping bolts 36 in the clamping bar.
It is to be understood that simplex orifice fittings F require routine maintenance and may also require that the orifice plate 10 be replaced with another orifice plate 10 having a bore 12 of different diameter. Replacement of the orifice plate 10 for one with a different size of bore 12 diameter may be required when the fluid flow rate is being changed. When it is desirable to inspect, repair, replace or otherwise service the orifice plate 10, the fluid pressure in the pipe is abated and the plate carrier 14 with the orifice plate 10 and seal ring 16 are manually removed from the simplex orifice fitting F. If the sealing gasket 22 needs replacing, the bolts 20 and the plate carrier 14 are removed from the sealing bar 18 and a new sealing gasket 22 is installed. The plate carrier 14 is manually bolted on to the sealing bar 18 with the sealing gasket 22 therebetween. After performing the necessary repair, replacement or maintenance operations, the assembly is again manually installed in the simplex orifice fitting F and manually tightened down. The final positioning of the orifice plate bore 12 along the X-axis is dependent on the maintained "trueness" of the line-up pin 26, the amount of "play" between the outer diameter of the line-up pin 26 and the inner diameter of the line-up hole 18b, and the "play" between the outer diameter of the alignment pin 30 and the inner diameter of the blind bores 14a and 18a. The final positioning of the orifice plate bore 12 along the Y-axis is dependent on the thickness of the sealing gasket 22 and the amount of compression of the sealing gasket 22 resulting from the torque the technician applies to the plurality of clamping bolts 36 in the clamping bar 34 and also dependent on the amount of compression of the sealing gasket 22 when bolting the plate carrier 14 to the sealing bar 18.
It is desirable to have a simplex orifice fitting which ensures that the orifice plate bore is concentric with or within accepted nominal eccentricities with the longitudinal bore of the simplex orifice fitting. It is also desirable that the simplex orifice fitting maintain nominal eccentricity following repeated plate carrier removal as required to inspect, repair, replace or otherwise service the orifice plate. It is desirable that the simplex orifice fitting be economical and dependable to ensure compliance with the tight tolerances required by API/AGA 14.3. It is further desirable that the simplex orifice fitting be able to repeatedly align the orifice plate bore substantially independent of the technician and external condition of the simplex orifice fitting.